[meteorite-list] The Magnetic Personality of Ancient Mars

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed May 11 16:33:53 2005
Message-ID: <200505112033.j4BKXG614611_at_zagami.jpl.nasa.gov>

http://www.space.com/scienceastronomy/050511_magnetic_mars.html

The Magnetic Personality of Ancient Mars
By Robert Roy Britt
space.com
11 May 2005

A new study of old rocks on Earth could force a revision of theories
about Mars. The results suggest ancient Mars might have been more
magnetic than thought, challenging basic assumptions about the evolution
of the red planet.

Unlike modern Earth, Mars has almost no magnetic field
today. Evidence has suggested Mars didn't have a very strong magnetic
field early on, either.

Our planet's magnetism is created by the rubbing of a solid inner core
against a liquid outer core, which rotate at different rates
and act as a dynamo. The magnetic field helps deflect cosmic radiation
and solar particles, making Earth comparatively more habitable.

Fossil compass

Magnetism is recorded in the structure of rocks. Superheated material,
when it cools, takes on a structure parallel to the prevailing magnetic
field at the time.

A planet's magnetic activity changes over the eons, in part because a
young planet cools and solidifies as it ages, so ancient bedrock can
serve as a time capsule for magnetism, a sort of fossil compass.

A study in 2003 found the core of Mars, at least the outer part, is
liquid.

Surveys in the 1990s of magnetic fields on Mars, by the orbiting Mars
Global Surveyor, detected the signatures of relatively intense magnetism
in some of the planet's more modern surfaces. But the fields were found
to be very weak in two large and old impact basis, called Hellas
and Argyre.

Each basin, carved out by a colossal space rock, is more than 3 billion
years old. The data implied that Mars had a weak magnetic field back then.

That analysis has influenced theories of how Mars cooled after its
formation and when its inner layers developed distinct boundaries.

Up close

The new research calls into question the validity of measuring magnetism
from an orbital perch.

A team led by Stuart Gilder of the Paris Earth Physics Institute found
that rocks in the 2-billion-year-old Vredefort impact crater in South
Africa -- the oldest such structure on Earth -- are highly magnetized,
yet from above the magnetism appears weak. Two other ancient craters
reveal similar differences.

The basic reason is simple: While magnetism is strong in individual
rocks, the direction varies from rock to rock in these impact craters,
so when examined from a distance, they cancel each other out.

The study is detailed in the May 12 issue of the journal Nature.

"Meteorite craters can then seem to be magnetic or non-magnetic,
depending on how close the magnetometer is to the source," writes David
Dunlop, a University of Toronto researcher, in an accompanying analysis.
"Viewed from satellite altitudes of 100-400 kilometers [60-250 miles],
martian impact basins would appear magnetically featureless if the
magnetic vectors of their source rocks vary in direction over distances
of a few kilometers or less."

Exactly why the rocks are magnetized randomly is more complicated.

Based on differing mineral structures in the rocks, Gilder and his
colleagues hypothesize that when a space rock hits, the shock of the
event would briefly create intense localized magnetic fields. Rocks that
cool during this initial period would be magnetized with orientation
related to these temporary field. Other rocks would cool more slowly,
and would take on the planet's magnetic orientation.
Received on Wed 11 May 2005 04:33:15 PM PDT